1999 Cadillac STS Engine Replacement

First thought, a replacement engine is not going to get you a roll back in miles, that is illegal. You say they tried to put a 98 engine in, did they succeed? It is possible that the oil life wasn't reset for one reason or another(forgot to, or didn't know how). The cold air warm air issue could easily be a dual zone climate control fault. On the alleged repair, who was it that recommended the engine be replaced with a used one, and who was responsible for providing it, you or the shop? It is very probable that the replacement engine has leaking head gaskets as well. If they tried to repair your original engine, there would have been concerns with head bolts that would not torque. It is the head bolt threads in the block that fail over 90% of the time, and the proper repair would involve thread inserts being installed. Identifying the engine may not be easy, it is dependant on how familiar you are with your vehicle under the hood, and how thorough and detail oriented the tech that performed the repairs was. Contrary to parts being interchangeable, there is not that much difference. Valve diameter/lift/duration, and PCM strategy are about all that seperate VIN 9 from VIN Y.